Zika-Linked Birth Defects Up in Parts of U.S. in Last Half of 2016

Action Points

Note that this observational epidemiologic study suggests that the types of birth defects often associated with Zika virus have increased in the U.S., particularly in states where Zika is present.

Be aware that this may not be capturing the full effects of Zika infection, as developmental problems can occur after birth.

Birth defects most strongly linked to congenital Zika virus infection increased 21% in U.S. states and territories with local transmission of the virus in the latter part of 2016, CDC researchers found.

After excluding neural tube defects and other early brain malformations, there was a statistically significant increase in prevalence of birth defects potentially linked to Zika in the last versus the first half of 2016 (2.0 per 1,000 live births versus 2.4 per 1,000, respectively, P=0.009) — for a total of 29 more cases than expected given the first half prevalence, reported Augustina Delaney, PhD, of the CDC, and colleagues.

There was an increase in total birth defects from the first to the second half of 2016, but it was non-significant (2.8 versus 3.0 per 1,000 live births, P=0.10), the authors wrote in the Morbidity and Mortality Weekly Report.

They argued that this increase “underscores the importance of surveillance for birth defects potentially related to Zika virus infection and the need for continued monitoring in areas at risk for Zika.” However, the team also noted that the overall prevalence numbers were similar to prior reports of baseline prevalence of birth defects potentially associated with Zika in 2013-2014. But, the researchers added, this report includes 12 more jurisdictions, representing about a quarter of all live births in U.S. states and territories.

A total of 15 states and U.S. territories were included in the report, which were divided into three groups:

Those with local Zika transmission in 2016

Those with at least one case of confirmed, symptomatic, travel-associated Zika infection in 2016

Those with less than one travel-associated Zika case per 100,000 residents reported to CDC in 2016

Overall, there were 2,962 infants and fetuses (92% were live births) with birth defects potentially related to Zika virus infection, and about half had brain abnormalities or microcephaly. Twenty-two percent of birth defects were “other consequences of [central nervous system] dysfunction without mention of brain or eye abnormalities,” the researchers reported. Neural tube defects comprised 20% of these birth defects, while eye abnormalities with no mention of brain abnormalities accounted for 9%. About 95% of these infants and fetuses did not have Zika testing performed or there were no results available.

Jurisdictions with local Zika transmission were defined as Puerto Rico, southern Florida, and Texas Public Health Region 11. There were 393 infants and fetuses in areas with local transmission with birth defects potentially related to Zika transmission, and three-quarters of those infants and fetuses did not receive Zika testing or had no test results available.

In a separate commentary in the Journal of the American Medical Association, CDC director Brenda Fitzgerald, MD, and colleagues argued that there was a significant increase in the prevalence of birth defects with the strongest link to Zika, which represent more than 90% of Zika-associated birth defects in the U.S. Zika Pregnancy and Infant Registry.

Fitzgerald and co-authors also warned that these numbers, while concerning, may represent only a portion of the full effect of congenital Zika virus infection; the team cited previous case reports of postnatal microcephaly and developmental difficulties that present as a child ages, highlighting the need for increased surveillance of the virus.

“Many infants with congenital Zika virus infection and severe microcephaly will have profound developmental delays and face significant challenges, but ongoing surveillance is needed to determine the extent to which congenitally exposed infants without apparent birth defects will experience similar or other developmental issues,” Fitzgerald and colleagues wrote.

Delaney et al noted several limitations — namely, that the three jurisdictions with local Zika virus transmission were very different in size and scope of transmission, and that the high prevalence of travel-related Zika virus could have contributed to the prevalence in these areas. In addition, the team said, increased awareness of birth defects may have resulted in increased identification.